Adds authentication and pricing pages for user onboarding

Introduces comprehensive user authentication flow with plan selection and registration

Includes:
- Pricing page with plan details and selection
- Payment page with plan summary and card information
- Signup page for different plan tiers
- Shared authentication layout and design system improvements

Enhances user onboarding experience with clear plan information and streamlined signup process

PlanTempus.Application/wwwroot/css/page.css

@@ -25,8 +25,8 @@ swp-page-header {
 }
 
 swp-page-title h1 {
-  font-size: 24px;
-  font-weight: 600;
+  font-size: var(--font-size-2xl);
+  font-weight: var(--font-weight-semibold);
   color: var(--color-text);
   margin-bottom: var(--spacing-1);
 }
@@ -65,12 +65,12 @@ swp-card-title {
   align-items: center;
   gap: var(--spacing-2);
   font-size: var(--font-size-base);
-  font-weight: 600;
+  font-weight: var(--font-weight-semibold);
   color: var(--color-text);
 }
 
 swp-card-title i {
-  font-size: 20px;
+  font-size: var(--font-size-xl);
   color: var(--color-text-secondary);
 }
 
@@ -127,12 +127,12 @@ swp-ai-header {
   gap: var(--spacing-2);
   margin-bottom: var(--spacing-2);
   font-size: var(--font-size-sm);
-  font-weight: 500;
+  font-weight: var(--font-weight-medium);
   color: var(--color-purple);
 }
 
 swp-ai-header i {
-  font-size: 16px;
+  font-size: var(--font-size-base);
 }
 
 swp-ai-text {
@@ -172,7 +172,7 @@ swp-quick-action-btn:hover {
 }
 
 swp-quick-action-btn i {
-  font-size: 18px;
+  font-size: var(--font-size-lg);
 }
 
 /* ===========================================